Various types of sheet conveying rollers are incorporated in sheet conveying mechanisms provided, for example, in electrostatic copying machines, laser printers, plain paper facsimile machines, copier-printer-facsimile multifunction machines, inkjet printers, automatic teller machines (ATM) and the like.
Examples of the sheet conveying rollers include sheet feed rollers, transport rollers, platen rollers and sheet output rollers, which are each adapted to be rotated in frictional contact with a sheet (the term “sheet” is herein defined to include a paper sheet, a plastic film and the like, and this definition is effective in the following description) to convey the sheet.
Such a conventional sheet conveying roller typically includes a tubular elastic member made of a rubber such as a natural rubber (NR), a urethane rubber (U), an ethylene-propylene-diene rubber (EPDM), a polynorbornene rubber, a silicone rubber or a chlorinated polyethylene rubber, and having an outer peripheral surface serving as a contact surface to be brought into contact with the sheet.
However, paper dust and dirt generated from paper sheets are liable to adhere to the outer peripheral surface of the elastic member. As the sheet conveying roller is repeatedly brought into contact with the paper sheets, the paper dust and the dirt are accumulated on the outer peripheral surface. This reduces the contact area and the frictional coefficient of the elastic member with respect to a paper sheet, thereby causing sheet transportation failure at a relatively early stage.
Particularly, paper sheets having a higher ash content are widely used for reduction of running costs of the machines in recent years. The higher ash content paper sheets are more liable to generate paper dust and dirt, so that the accumulation of the paper dust and the dirt and the sheet transportation failure attributable to the accumulation are more liable to occur.
Various attempts have been made to prevent the accumulation of the paper dust and the dirt, and the sheet transportation failure attributable to the accumulation.
Patent Literature 1, for example, discloses a so-called knurled roller, which includes a non-porous tubular elastic member such as of a rubber having a plurality of grooves formed in an outer peripheral surface thereof as extending parallel to its axis along its entire width for use as a sheet conveying roller.
The grooves of the knurled roller function as pockets for trapping paper dust and dirt, thereby reducing the amount of the paper dust and the dirt accumulated on the outer peripheral surface to be brought into direct contact with paper sheets. Thus, the sheet transportation failure can be suppressed which may otherwise occur due to the accumulation of the paper dust and the dirt.
However, the knurled roller suffers from the following problem, because the grooves extend axially of the elastic member along the entire width of the elastic member.
Consider a state in which a region of the outer peripheral surface of the elastic member formed with the groove is in contact with a sheet and a state in which a region of the outer peripheral surface of the elastic member formed with no groove is in contact with a sheet. A sheet contact area is smaller in the former state than in the latter state and, hence, a sheet transportation force is smaller in the former state than in the latter state.
Therefore, the two types of regions having different transportation forces are alternately brought into contact with the sheet, as the knurled roller is rotated. Accordingly, the sheet transportation force is pulsated. This makes it impossible to maintain the transportation force at a constant level.
Consider a state in which opposite edges of the groove abut against the sheet. In this state, a distance between the center axis of the knurled roller and the sheet is smaller than in the state in which the region of the outer peripheral surface formed with no groove abuts against the sheet. That is, the knurled roller virtually has different outer diameters in the region thereof formed with the groove and in the region thereof formed with no groove.
Therefore, the two types of regions having different outer diameters are alternately brought into contact with the sheet, as the knurled roller is rotated. Accordingly, the sheet transportation speed is also pulsated. This makes it impossible to maintain the transportation speed at a constant level.
Further, the region of the outer peripheral surface formed with no groove transports the sheet in surface contact with the sheet, while the region of the outer peripheral surface formed with the groove transports the sheet with the edges of the groove brought into line contact with the sheet by a so-called scratching effect. Where paper sheets having a greater amount of paper dust and dirt are occasionally transported, for example, the grooves extending along the entire width of the knurled roller simultaneously lose the scratching effect along their entire lengths, so that the sheets are liable to suffer from slippage.
Therefore, the knurled roller liable to suffer from the pulsation of the transportation speed and the slippage is not suitable for a use application requiring precise sheet feeding or for a use application requiring the control of the sheet feed amount for the sheet feeding.
In addition, a sheet contact pressure does not act on a groove portion of the knurled roller and, if the sheet contact pressure is set at a higher level, there is a great difference in contact pressure between the groove portion and portions of the knurled roller located at opposite sides of the groove portion, resulting in wrinkle or indentation of the sheet.
Patent Literature 2 discloses a sheet conveying roller including a porous elastic member produced as having a multiplicity of cells by vulcanizing and foaming a rubber composition containing a foaming agent, and grinding an outer peripheral surface of the elastic member to expose multiple cells adjacent to the outer peripheral surface to form openings.
In the sheet conveying roller, the openings function as pockets for trapping paper dust and dirt, thereby reducing the amount of the paper dust and the dirt accumulated on the outer peripheral surface to be brought into direct contact with the paper sheet. Thus, the sheet transportation failure attributable to the accumulation of the paper dust and the dirt can be suppressed.
However, the porous elastic member has a smaller density, and is liable to be worn or suffer from a so-called permanent compressive deformation which is a phenomenon such that the elastic member is not restored to its original diameter after being compressed for a long period of time. Therefore, the sheet conveying roller including the porous elastic member is more liable to suffer from the wear and the permanent compressive deformation and, hence, has a shorter product service life than a sheet conveying roller including a non-porous elastic member.
If the outer diameter of the sheet conveying roller is changed due to the wear or the permanent compressive deformation, the sheet feed amount is significantly reduced as compared with the initial state even if the sheet conveying roller is rotated at the same rotation speed. That is, the sheet feed amount is not constant, but constantly changed due to the wear and the permanent compressive deformation of the elastic member.
Therefore, the sheet conveying roller including the porous elastic member is not suitable for a use application requiring precise sheet feeding or for a use application requiring the control of the sheet feed amount for the sheet feeding.
The shapes, the dimensions and the density (the per-unit-area number) of and distances between the openings formed in the outer peripheral surface, for example, by the foaming of the foaming agent and the subsequent grinding are not uniform, but vary on the outer peripheral surface of the elastic member. If the number of the openings is locally reduced or the diameters of the individual openings are locally reduced in some region on the outer peripheral surface, for example, the openings in the region fail to sufficiently function as the pockets for trapping the paper dust and the dirt, thereby failing to sufficiently suppress the sheet transportation failure attributable to the accumulation of the paper dust and the dirt. The variation in the geometry of the openings varies from product to product, so that the sheet transportation failure suppressing function also varies from product to product.
Patent Literature 3 discloses an elastic member which includes a plurality of annular grooves provided in an outer peripheral surface thereof and arranged perpendicularly to the axis thereof as extending circumferentially of the outer peripheral surface.
However, the grooves extending circumferentially of the outer peripheral surface fail to sufficiently function as pockets for trapping paper dust and dirt as compared with the aforementioned axial grooves and the openings of the porous cells.
In the two types of sheet conveying rollers previously described, the paper dust and the dirt adhering to the outer peripheral surface of the elastic member are conveyed to the axial grooves or the openings of the porous cells by the rotational force applied mainly circumferentially of the outer peripheral surface, and trapped in the axial grooves or the openings of the porous cells.
However, the circumferential grooves extend generally parallel to the direction of the rotational force, thereby failing to efficiently convey and trap the paper dust and the dirt by the rotational force.
Therefore, the circumferential grooves fail to sufficiently function as the pockets for trapping the paper dust and the dirt.
In addition, a sheet contact pressure does not act on a groove portion of the sheet conveying roller and, if the sheet contact pressure is set at a higher level, there is a great difference in contact pressure between the groove portion and portions of the sheet conveying roller located at opposite sides of the groove portion, resulting in wrinkle and indentation of the sheet.